Variable induction magnets of the type used in synchrotrons



J. PARAIN Sept. 25, 1962 4 SheetsSheet 1 Filed June 11, 1957 INVENTOR M60055 PARA/ll BY 9 6. OwW/LM/ ATTORNEY Sept. 25, 1962 PARA|N VARIABLE INDUCTION MAGNETS OF THE TYPE USED I Filed June 11, 1957 N SYNCHROTRONS 4 Sheets-Sheet 2 FIG. 6

INVENTOR M50055 P/lfi4/A/ BY A, GJKW4W4 ATTORNEY Sept. 25, 1962 Filed June 11, 1957 v I J PARA N 3,056,069. VARIABLE INDUCTION MAGNETS OF THE TYPE USED IN SYNCHROTRONS 4 Sheets-Sheet 3 FIG. /0

F [6. II I K=2 L8 L72 L6 L4 A 20 K. guuss B;

INVENTOR M60055 PARA/4 ATTORNEY p 25, 1962 J. PARAlN 3,056,069

VARIABLE INDUCTION MAGNETS OF THE TYPE USED IN SYNCHROTRONS Filed June 11, 1957 4 Sheets-Sheet 4 INVENTOR J46 01/55 PARA/N ATTORNEY Patented Sept. 25, 1962 3,056,069 VARIABLE INDUCTION MAGNETS OF THE TYPE USED IN SYNCHROTRONS Jacques Parain, Paris, France, assignor to Commissariat a IEnergie Atomique, Paris, France, a French state administration Filed June 11, 1957, Ser. No. 665,064 Claims priority, application France June 13, 1956 Claims. (Cl. 317-158) The present invention relates to variable induction magnets of the type used in synchrotrons.

The object of this invention is to provide a magnet of this type in which the deformations of the field in the air gap due to magnetic saturation under the effect of high induction are corrected in a simple and efficient manner.

The essential feature of this invention consists in the fact that, in the portion of at least one of the pole pieces of the magnet Which is adjoining the air gap, there is provided a Zone the permeability of which is different from that of the material which constitutes the remainder of said pole piece, and is lower than it, so as automatically to correct at least partly the deformations of the magnetic modified for relatively low values of the induction.

It should be remembered that in apparatus such as synchrotrons the induction in the air gap increases during the whole particle acceleration cycle and that it is generally desired to maintain, during this cycle and at every point of the useful zone of the air gap, the gradient ZR of the induction in the radial direction as constant as possible.

For the sake of clarity, the term index of the magnetic field will be used to designate the quantity:

te F, dR where R is the radius of the stable orbit of the particles (which is constant for a given apparatus), and B is the induction along this orbit; this index n ranges from 0.5 to 0.75 in conventional synchrotrons.

As the induction in the air gap may generally vary from 300 to 15,000 gausses for instance, it is not possible to avoid, for values of the induction above approximately 10,000 gausses, a saturation of the magnetic circuit of the magnet (yoke and pole pieces) which deteriorates the and leads to a substantial increase of index n.

The solutions generally adopted to obviate these drawbacks will be summed up with reference to FIGS. 1 to 3.

FIG. 1 is a diagrammatic sectional view of a magnet including yoke 1, pole pieces 2 and 3 removably fixed on said yoke and excitation windings 4 and 5.

For low values of the induction, the faces 6 and 7 of pole pieces 2 and 3 are equipotential surfaces of the mag the induction gradually faces of the pole pieces; they become oblique thereto, as shown at 8 and 9, which has for its effect to modify the field in the air gap in an undesirable manner.

The curves of FIG. 2 illustrate the modification of the field in the air gap of a synchrotron magnet. In said FIG. Zthevalues of the index n of the field are plotted along the ordinates, and for points of the air gap the positions of which, with respect to the pole pieces, are plotted along the abscissa. R is the radius of the stable orbit, the radii increasing from left to right in the direction of arrow R.

FIG. 2 shows curves 10, 11, 12, 13, 14 and 15 representing the variations of the field index n at different points of the air gap for the following values of the induction B Curve 10-from 3,000 to 7,000 gausses, Curve 11 l0,000 gausses,

Curve 12-12,000 gausses,

Curve 13- l3,000 gausses,

Curve I l-14,000 gausses and Curve 15 15,000 gausses.

of the field index n is due to the fact that the difference of consumption of ampere-turns in the yoke along two induction lines of different total lengths, such as 16 and 17 on FIG. 1, has a greater influence for high values of the induction.

If, now, it is desired to stance 0.75, the curves of FIG. 2 show that the limit of tuted by conductors placed in the air gap and parallel to the stable orbit of the particles. This is in particular statled problem which is simple and avoids such drawbac s.

placement of the equipotential surfaces and 7 on FIG. 1. 1

The form of this correction zone and the permeability of this zone are determined in accordance with the desired chart of the field to be obtained.

One of the main advantages of this arrangement according to the present invention is that the correction of index n is thus automatically obtained and does not require, as in the case of. correcting windings, the provision of means for controlling the correction as a function or the induction. Furthermore, the fictitious increase of the air gap for high values of the induction, due to saturation of the correction zone, is much smaller than the space necessary for housing the correcting windings which otherwise would have to be used. This ensures a substantial saving of the excitation ampere-turns of the magnet.

Correction of the defects of the field index for high values of the induction by creating a correction zone in the pole piece therefore permits of eliminating the drawbacks of correcting windings and of increasing the potentialities of a magnet intended to work with a variable induction (synchrotron or mass separator magnet for instance).

The defects of the magnet resulting from magnetic saturation may thus be corrected according to the invention in two difierent manners, corresponding to two different causes;

(a) the increase An of the field index n due to saturation of the whole of the magnetic circuit, when the induction increases from its initial value (B (unsaturated circuit) to its maximum value (B is compensated for by providing correction zones of variable thickness,

(b) the effect of the projecting edges (hereinafter called horns) of the pole pieces, which is less important than the first mentioned defect, is compensated for by providing correction zone of uniform height in the central portion of the pole pieces.

Preferred embodiments of my invention will be hereinafter described with reference to the appended FIGS. 4 to 15 exclusive, given merely by way of example and in which:

FIGS. 4 and 5 are diagrammatic cross sections of pole pieces made according to the invention to correct the two above mentioned defects respectively.

FIGS. 6 and 7 on the one hand, and FIGS. 8 and 9, on the other hand, give curves showing the influence upon induction B and index n of the correction zones illustr-ated by FIGS. 4 and 5 respectively.

FIG. 10 is a part perspective view of the suriace of a pole piece, provided with notches, made according to my invention, said pole piece being intended to be used in a synchrotron.

FIG. 11 shows curves giving the equivalent permeability p. of the correction zone as a function of the induction B and of the dimensions of the above mentioned notches.

FIG. 12 shows curves giving function of the same parameters.

FIG. 13 shows curves giving the values of index n as a function of the radius R in the air gap.

FIGS. 14 and 15 diagrammatically illustrate modifications of the correction zone according to my invention.

On FIG. 4, the correction zones and 21 are of varying thickness from one edge to the other of the air gap so as to compensate for the increase of the field due to saturation of the whole of the magnetic circuit. FIG. 5 shows correction zones 26 and 27 of uniform thickness used to compensate for the effect of the saturation of the pole piece horns 22, 23, 24 and 25.

As a matter of fact, the two above mentioned defects exist simultaneously in the magnet and the correction zone that is actually used is either a combination of the arrangement of FIGS. 4 and 5 or possibly an arrangement according to only one of these figures. FIG. 6 shows curves obtained by plotting in ordinates the induction in the air gap at different points and in abscissas the radial distances of said points. On FIG. 7 the field index in the air gap is plotted in ordinates and the radial distances in abscissas.

the variation An as a On FIG. 6, curve 28 shows the initial induction (B (the magnetic circuit being unsaturated) and curve 29 shows the maximum induction (B (the magnetic circuit being saturated); the values of the induction are measured with units equal to B The curve 30 is obtained after correction, which curve is substantially the same for all values of induction B The cross-hatched area clearly shows the effect of the correction zone of variable height. FIG. 6 also shows that, in the central portion of the pole piece where curves 28 and 30 are substantially rectilinear, said curves have the same slope which means that index n, according to the invention, has been kept practically constant for the whole range of variations of the induction, that is to say from (B to o)m- FIG. 7 shows straight lines 31 and 32 which respectively represent the values of index n for inductions equal respectively to (B and (B when there is no correction zone; the difference between these two values of index n is equal to the above defined quantity An. When a correction zone according to the invention is provided, the field index n remains at its initial value (curve 31) for all values of the induction, even (B In a likewise manner, FIG. 8 shows curves where the induction in the air gap has been plotted in ordinates and the radial distances in abscissas. FIG. 9 shows in ordinates the field index in the air gap and in abscissas the radial distance.

On FIG. 8, 33 is the curve corresponding to the value (B of the induction (pole piece horns being unsaturated) =and 34 is the curve corresponding to the maximum value (B of the induction (pole piece horns being saturated). In this case also, the values of the induction are relative and equal to Furthermore, for the sake of clarity, these figures correspond to the case where the pole pieces of the magnet are parallel (11:0). The curve 35 is obtained after correction, which curve is substantially the same {or all values of induction B The cross-hatched area clearly shows the effect obtained by means of a correction zone of uniform thickness.

Curve 34 is substantially a parabola, and consequently curve 36 of FIG. 9 which represents, for induction (B,,),,,, the variations of index n as a function of radius R, is practically a straight line. The following, An will designate the variations, measured at the limit of the useful zone having a half length equal to r, of the field index n due only to saturation of the pole piece horns (FIG. 9).

In these conditions, a mathematical study oi the problem shows that a being the permeability of the correction zone, supposed to be made of the same material for both correction zones, and 1. being lower than the permeability [1. of. the remainder of the pole piece, the angle A5 of the correction zones 20 and 21 of FIG. 4 is given by the following formula: E

I (I) AB An 2R0 p, I in which E is the dimension of the air gap.

Angle Afi must of course be of a direction such that it produces a correction zone opposed to the defect to be corrected.

This angle A5 being constant for a given pole piece, same as the quantity i it follows that for any value of induction B- there is a relation such as: (H) An-p.=A in which A is a constant for a given angle Afi.

This relation (I I) is very important because, since the curves of FIG. 2 give the value of An as a function of B the law of variation of as a function of said induction B is thus known, which practically determines the choice of the material of which the correction zone is made.

In the case of a magnet to be established, it is of course not possible experimentally to plot the curves of FIG. 2, and use is made, in order to find the law of variation of ,u' as a function of B of the known relation:

Arugu =D in which: ,u is the value of the permeability in the yoke, D is a constant for a given magnet.

As for the correction zones 26 and 2.7 of uniform height, they are characterized by their thickness Ae (FIG. 5) the value of which is given by the following formula:

(III) Ae=-.B o.. in which Ab is the reduction of induction that must be produced at a point of radius R,, (FIG. 8).

In the particular case where a linear approximation is considered as sufficient (FIG. 9), Formula III becomes r (FIG. 5) being the half width of the useful zone and 0 (FIG. 4) the half width of the correction zone of variable thickness. This Formula V gives a thickness Ah which is always very small.

FIGS. 10 to illustrate different embodiments of the present invention as applied to variable induction magnets of the type used in synchrotrons.

In the preferred embodiment of the invention shown by FIG. 10, the correction zone consists partly of iron and partly of air so as to comply with condition (H). For this purpose, notches are provided in pole piece 37 by juxtaposing metal sheets of different outlines, 38 being the initial outline of the pole piece.

For rectangular notches equivalent permeability ,u'

fi E (VI) k (VII) B=n-% in which: k is -a geometrical coefiicient such that +f lc=- (FIG. 10), a being the width of every notch and f the distance between two consecutive notches,

this purpose, for a given value of k, B is chosen at will so that, knowing the curve F (B for the iron that is used, first and then ,u' are deduced by means of Formula VI. Formula VII then gives B so that one point of the curve of FIG. 11 is obtained.

The determination of A5, A2 and k permits subsequently of establishing a correction zone in the pole piece.

The influence of k is illustrated by the curves of FIG. 12 which shows as a function of induction B in dotted lines, the curve 43 representing the variations of An on the stable orbit in the absence of any correction and in solid lines the curves 44 and 45 representing the variations An due to the presence of correction zones according to the invention for two respective values of k (k=1.4 and k= 2) The value of Ali in each of these two cases has been chosen in such a manner as to compensate exactly for the variation An of index u when the induction is equal to 14,000 gausses.

It will be seen that the value k=1.4 produces too great a correction for inductions higher than 14,000 gausses (curve 44, FIG. 12) and that the value k=2 does not give a suflicient correction for values of the induction above 14,000 gausses (curve 45, FIG. 12).

By then plotting the correction surfaces for different values of k, it is found that k=l.72 gives a satisfactory correction of the field index over the whole cycle (curve 46, FIG. 12).

In these conditions, when the induction is equal to 15,000 g-ausses ,u=5.2 (value given by curve 47, FIG. 11).

In the example described:

c=d=250 mm. An=0.66 for r: mm. (curve 36, FIG. 9)

(Formula IV) The increase of the consumption of ampereturns of excitation of the magnet corresponds to a height of air Ah:

Ah=2.06 10- Ct490 250+$ X 180)=3.8 mm.

(Formula V) whereas for the same magnet, correcting windings would occupy 16 mm. of the air gap. The consumption of the correction zone is thus about only 2% of the total consumption of the machine.

It is possible, with the correction device according to the invention, to operate the magnet under an induction of 15,000 gau-sses without any reduction of the efliciency of the correction. Curves 48, 49, 50, 51, 52 and 53 of FIG. 13 represent, in an improved magnet according to the invention, the variations of index n as a function of the radius for the same values of the induction as in FIG. 2 (that is to say respectively from 3,000 to 7,000 gausses, and for 10,000, 12,000, 13,000, 14,000 and 15,000 gausses) and show by comparison with said FIG. 2. the efficiency of the correction zone thus obtained.

In another embodiment of the invention illustrated by FIG. 14, a more accurate correction is obtained by varying the angle AB radially, every value of A5 (A5 A8 A serving to correct corresponding defects A11 An For this purpose, pole piece 54 is divided into three elementary portions 55, 56 and 57 for each of which the index variation An and the corresponding angle A13 have been determined by means of the above Formula I.

In a genenal manner, while I have, in the above description, disclosed what I deem to be practical and efficient embodiments of my invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts Without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

For instance, the correction, instead of being obtained by providing radial notches, might be achieved by making use of different geometrical arrangements (holes transverse grooves in the pole piece) or by making use of a homogeneous material such as a sintered alloy. In this case, the correction zone would consist of a plate 58 (FIG. 15) fixed on the pole piece.

For low values of the induction, the defects due to the remanant field may also be corrected by a correction zone according to the invention.

What I claim is:

1. In a synchrotron, of a type having an axis about which a stream of particles orbit, a variable induction magnet of the type for establishing inductions varying from very high values, close to iron saturation value, to

relatively low values, comprising in combination;

a substantially annular yoke of magnetic material having said axis as a center and a substantially C-shaped cross section along its annular length,

annular pole pieces in opposed face to face relationship terminating the ends of said yoke and having substantially parallel surfaces defining an air gap therebetween in which the orbit of said particles falls,

an annular correction zone substantially co-extensive with at least one of said pole pieces of a magnetic permeability lower than that of the material of which the remainder of said yoke is made,

said annular correction Zone partially defined by an annular inner edge closest to said axis,

and an annular outer edge furthest from said axis, the thickness of said correction zone defined by one 6 is said angle; E is the distance between said parallel surfaces; R is the radius between said axis and said orbit; and being the permeability of the material of which said correction zone is made; and

E dR

where B is the value of the magnetic field along said orbit, and the 1 dR is the derivative of the magnetic field value with the orbit radius,

least partly corrects the deformafield in said air gap for high values whereby said zone at tions of the magnetic of induction.

2. The induction magnet defined in claim '1 wherein the depth of said zone varies continuously from said first edge to said second edge. 4

3. The induction magnet defined in claim 1 wherein the depth of said zone is determined by a plurality of connecting planes each of which are angularly disposed at different angles to said axis.

References Cited in the file of this patent UNITED STATES PATENTS Germany 

